Microwaves can be generated as direct radiation from electrical sparks across gaps at high electric potential. The spark gap can also be a part of a very high frequency oscillating circuit which radiates electromagnetic waves. Microwaves can also be derived from the thermal radiation of warm bodies. Some important microwave generators are klystrons, magnetrons, and traveling wave tube oscillators. Their power outputs range from microwatts to thousands of kilowatts, depending upon the type and design of the generator, the operating frequency and pulse duration.
It is known that microwaves can be produced from an oscillating virtual cathode in a device called the vircator. See for example, H. Sze, et al., "A Radially and Axially Extracted Virtual Cathode Oscillator (Vircator)," IEEE Transactions on Plasma Science, Vol. PS-13, No. 6, December 1985. In the vircator, microwave generation takes place when the injected current of an electron beam machine exceeds the space-charge limiting current of the region behind the anode-cathode gap. When the limiting current is exceeded, electrons accumulate in this region forming a virtual cathode which causes some electrons to be reflected. It is known that the frequency, or frequencies, of the generated microwaves is determined by the transit time of the reflected electrons and the oscillation frequency of the virtual cathode.